NAME

SYNOPSIS

DESCRIPTION

This is an implementation of the User Datagram Protocol
described in RFC 768.
It implements a connectionless, unreliable datagram packet service.
Packets may be reordered or duplicated before they arrive.
UDP generates and checks checksums to catch transmission errors.

When a UDP socket is created,
its local and remote addresses are unspecified.
Datagrams can be sent immediately using
sendto(2)
or
sendmsg(2)
with a valid destination address as an argument.
When
connect(2)
is called on the socket, the default destination address is set and
datagrams can now be sent using
send(2)
or
write(2)
without specifying a destination address.
It is still possible to send to other destinations by passing an
address to
sendto(2)
or
sendmsg(2).
In order to receive packets, the socket can be bound to a local
address first by using
bind(2).
Otherwise, the socket layer will automatically assign
a free local port out of the range defined by
/proc/sys/net/ipv4/ip_local_port_range
and bind the socket to
INADDR_ANY.

All receive operations return only one packet.
When the packet is smaller than the passed buffer, only that much
data is returned; when it is bigger, the packet is truncated and the
MSG_TRUNC
flag is set.
MSG_WAITALL
is not supported.

IP options may be sent or received using the socket options described in
ip(7).
They are processed by the kernel only when the appropriate
/proc
parameter
is enabled (but still passed to the user even when it is turned off).
See
ip(7).

When the
MSG_DONTROUTE
flag is set on sending, the destination address must refer to a local
interface address and the packet is sent only to that interface.

By default, Linux UDP does path MTU (Maximum Transmission Unit) discovery.
This means the kernel
will keep track of the MTU to a specific target IP address and return
EMSGSIZE
when a UDP packet write exceeds it.
When this happens, the application should decrease the packet size.
Path MTU discovery can be also turned off using the
IP_MTU_DISCOVER
socket option or the
/proc/sys/net/ipv4/ip_no_pmtu_disc
file; see
ip(7)
for details.
When turned off, UDP will fragment outgoing UDP packets
that exceed the interface MTU.
However, disabling it is not recommended
for performance and reliability reasons.

Address format

Error handling

All fatal errors will be passed to the user as an error return even
when the socket is not connected.
This includes asynchronous errors
received from the network.
You may get an error for an earlier packet
that was sent on the same socket.
This behavior differs from many other BSD socket implementations
which don't pass any errors unless the socket is connected.
Linux's behavior is mandated by
RFC 1122.

For compatibility with legacy code, in Linux 2.0 and 2.2
it was possible to set the
SO_BSDCOMPATSOL_SOCKET
option to receive remote errors only when the socket has been
connected (except for
EPROTO
and
EMSGSIZE).
Locally generated errors are always passed.
Support for this socket option was removed in later kernels; see
socket(7)
for further information.

When the
IP_RECVERR
option is enabled, all errors are stored in the socket error queue,
and can be received by
recvmsg(2)
with the
MSG_ERRQUEUE
flag set.

/proc interfaces

System-wide UDP parameter settings can be accessed by files in the directory
/proc/sys/net/ipv4/.

udp_mem (since Linux 2.6.25)

This is a vector of three integers governing the number
of pages allowed for queueing by all UDP sockets.

min

Below this number of pages, UDP is not bothered about its
memory appetite.
When the amount of memory allocated by UDP exceeds
this number, UDP starts to moderate memory usage.

pressure

This value was introduced to follow the format of
tcp_mem
(see
tcp(7)).

max

Number of pages allowed for queueing by all UDP sockets.

Defaults values for these three items are
calculated at boot time from the amount of available memory.

udp_rmem_min (integer; default value: PAGE_SIZE; since Linux 2.6.25)

Minimal size, in bytes, of receive buffers used by UDP sockets in moderation.
Each UDP socket is able to use the size for receiving data,
even if total pages of UDP sockets exceed
udp_mem
pressure.

udp_wmem_min (integer; default value: PAGE_SIZE; since Linux 2.6.25)

Minimal size, in bytes, of send buffer used by UDP sockets in moderation.
Each UDP socket is able to use the size for sending data,
even if total pages of UDP sockets exceed
udp_mem
pressure.

Socket options

To set or get a UDP socket option, call
getsockopt(2)
to read or
setsockopt(2)
to write the option with the option level argument set to
IPPROTO_UDP.
Unless otherwise noted,
optval
is a pointer to an
int.

UDP_CORK (since Linux 2.5.44)

If this option is enabled, then all data output on this socket
is accumulated into a single datagram that is transmitted when
the option is disabled.
This option should not be used in code intended to be
portable.

Ioctls

Gets a pointer to an integer as argument.
Returns the size of the next pending datagram in the integer in bytes,
or 0 when no datagram is pending.
Warning:
Using
FIONREAD,
it is impossible to distinguish the case where no datagram is pending
from the case where the next pending datagram contains zero bytes of data.
It is safer to use
select(2),
poll(2),
or
epoll(7)
to distinguish these cases.

TIOCOUTQ (SIOCOUTQ)

Returns the number of data bytes in the local send queue.
Supported only with Linux 2.4 and above.

In addition, all ioctls documented in
ip(7)
and
socket(7)
are supported.

ERRORS

All errors documented for
socket(7)
or
ip(7)
may be returned by a send or receive on a UDP socket.

ECONNREFUSED

No receiver was associated with the destination address.
This might be caused by a previous packet sent over the socket.

VERSIONS

IP_RECVERR
is a new feature in Linux 2.2.

SEE ALSO

RFC 768 for the User Datagram Protocol.
RFC 1122 for the host requirements.
RFC 1191 for a description of path MTU discovery.

COLOPHON

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man-pages
project.
A description of the project,
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and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.